Muscles, tendons, and ligaments have spring-like characteristics. Because these musculoskeletal elements change length when joints flex or extend, it is not surprising that joints exhibit spring-like characteristics. The control of musculoskeletal stiffness is complex with many factors affecting the stiffness of each joint. However, in some multi-jointed movements, including mammalian running, the elements of the musculoskeletal system are integrated together so that the overall musculoskeletal system exhibits spring-like behavior. Experimental findings on the mechanics of running gaits have revealed that the overall musculoskeletal system behaves like a single linear spring in all of the mammals studied to date, including running humans, trotting dogs and horses, and hopping kangaroos. This observation has led to the development of a spring-mass model for running, consisting of a single linear massless "leg spring" and a mass. The "leg spring" represents the spring-like characteristics of the overall integrated musculoskeletal system during locomotion, and the mass is equivalent to the mass of the animal. The general objective of the proposed research is to gain an understanding of the link between musculoskeletal stiffness and locomotion biomechanics. Given the complexity of the control of the stiffness of a single muscle or joint, it is not realistic to use a forward dynamics approach that begins at the level of the stiffness-of a single muscle and attempts to explain the mechanics of running. We propose to use an inverse dynamics approach that begins by focusing on the link between locomotion mechanics and overall musculoskeletal stiffness. Under the umbrella of Specific Aim 1, the importance of adjustments to the stiffness of the overall musculoskeletal system to accommodate running on varied terrain is examined. This research will involve examining the adjustments to musculoskeletal stiffness for running on surfaces of different stiffnesses and surfaces of varying predictability. Under the umbrella of Specific Aim 2, we will do a series of interrelated studies examining the mechanisms for adjusting the stiffness of the overall musculoskeletal system during running. These studies will include examining the range over which the stiffness of a single joint of the leg can be adjusted during locomotion. In addition, it will involve examining the relative importance of changes to joint stiffness and posture in adjusting the stiffness of the overall musculoskeletal system during running. The experiments under both Specific Aims l and 2 will involve a combined kinetic and kinematic analysis of running to determine overall musculoskeletal stiffness and joint stiffness. The findings will give new information about the optimal design of floors and tracks for minimizing overuse injuries during sustained weight-bearing aerobic activities, and the optimal design of spring-based prosthetic legs and robotic legs. Finally, our research will begin applying knowledge of the neural control of joint stiffness to understanding the mechanics of a natural activity that is performed by all legged animals, locomotion.